1. Field of the Invention
The invention relates to an interconnection element for an asynchronous time-division multiplex digital transmission system wherein information cells which are incoming on input lines are transmitted to an output line by intermediate storage of the cells in buffers and evaluation in a comparator of the path identification contained in the cells.
2. Description of the Related Art
In an asynchronous time-division multiplex digital transmission system useful information, for example telephone, picture or sound signals, is transmitted in data blocks of a fixed length called cells, each cell having a predetermined number of bits. Each cell consists of a header field and an information field. The header field contains inter alia the path identification for the cell. Path identification should here be understood to mean connection identification giving path routing identification. The connection information comprises the data on the target or subtarget destination of the useful information. In certain transmission systems the path routing information is added within the system and contains data on a subtarget within the transmission system. The useful information is accommodated in the information field.
Consecutive time intervals (time frames) are assigned to consecutive cells. The duration of such a time interval depends on the clock frequency employed for the transmission system components. Even if no useful information is available, idle cells, i.e. cells without useful information, are transmitted in their assigned time frames. Cells containing useful information are designated useful cells.
During the transmission of the cells between subscribers, the cells pass through switching networks in which paths corresponding to the evaluation of the path identifications are established. Such a switching network may be composed of a plurality of switching network blocks. Such a switching network block having a plurality of input lines and output lines is comprised of a plurality of interconnection elements. An interconnection element has a plurality of input lines and an output line. In an interconnection element cells are then passed from the input line to an output line. On arrival of cells from a plurality of input lines during a time frame, wanting to access the output line, specific interconnection strategies are required.
The interconnection element defined above is known from published German application DE-OS 37 14 308. Therein the cells arriving on input lines are always stored in an input buffer. After evaluation of the path identification in a comparator (packet filter) assigned to each input line, the output line is enabled for those of the cells in the input buffer having a path identification in agreement with the address of the output line. If simultaneously several cells stored in the input buffers assigned to the input lines are to be provided to an output line, then a decision circuit cyclically enables each comparator assigned to an input buffer or an input line to output the cells from the relevant input buffers. Because of the fact that a comparator is assigned to each input line the interconnection element is expensive and complicated.